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Jeff Tollefson

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Summers in the northern hemisphere are growing hotter at a more rapid pace than winters are warming. credit: Dave Sanders/The New York Times/Redux/eyevine

An analysis of decades of satellite data has revealed how humans are changing seasonal cycles in the lower atmosphere. The accumulation of greenhouse gases produced by burning fossil fuels has increased air temperatures in summer and caused larger annual temperature swings in the northern hemisphere.

Previous research has documented how global warming is altering seasons on the ground — causing winter snowpacks to melt earlier, shifting animal migrations and lengthening fire seasons. The latest study, which draws on satellite temperature records from 1979 to 2016, is the first to pinpoint seasonal changes in the atmosphere. The odds that natural climate variability can account for the magnitude of the temperature changes over the course of the satellite record are roughly five in a million, researchers report on 19 July in Science1.

“In the biological world, lots of people have been looking for and finding these changes, so we decided to take a look at the satellite data,” says lead author Benjamin Santer, an atmospheric scientist at the Lawrence Livermore National Laboratory in California. “What we see is profound evidence of the human impact on climate, not only in the annual temperatures but also in the seasonal cycle.”

Santer’s team used computer models to simulate the global climate over thousands of years, with and without greenhouse-gas emissions. This allowed them predict telltale ‘fingerprints’ of human influence on atmospheric conditions. The researchers then compared those patterns with satellite temperature data collected since 1979. That revealed that natural variations in Earth’s climate could not account for all of the observed changes in seasonal climate cycles.

Some seasonal climate trends cannot be explained without including warming caused by rising atmospheric greenhouse-gas concentrations, the scientists found. The most significant change was in the northern hemisphere, where the difference between summer and winter temperatures has increased by around 0.4 °C, mostly due to the fact that summer temperatures are rising more quickly. This effect was most notable over land, perhaps due to increasingly dry land surface conditions.

Climate sceptics

The study is solid, if not surprising, says Andrew Dessler, an atmospheric scientist at Texas A&M University in College Station. “I don’t think this solves a major problem in atmospheric sciences, nor does it change anything that I think about the climate system,” Dessler says. “But it does provide even more evidence that humans are altering the climate.”

Santer says that one impetus for the analysis was to counter arguments from climate sceptics. Satellite temperature records have long been a source of both scientific intrigue and controversy, in part because they once registered less warming than did temperature measurements collected at ground-based weather stations around the world.

Much of that apparent discrepancy between satellite and surface temperature records has evaporated over the past two decades, as scientists corrected for errors caused by sensor calibrations2, transitions between satellites and gradual changes in the probes’ orbits over time. Still, Santer notes, some public officials who oppose taking action on climate change continue to incorrectly cite satellite temperature data.

In a paper published last year in Scientific Reports3, for instance, Santer and several colleagues rebutted a statement by Scott Pruitt, then the administrator of the US Environmental Protection Agency, who had said in congressional testimony that global warming had levelled off over the past two decades.

“If you are going to elevate satellite temperature data in public discourse, then let’s have a discussion about what the data tell us,” Santer says.